46 



THE CHEMISTRY OF LIFE 



tinuous phase, the water is in between in the discontinuous phase, and 

 the mixture is in the gel state. We can reheat the mixture and restore 

 the sol state. The white of an egg is in a liquid state when it comes 

 from the shell ; but sufficient heat will cause the colloidal particles to 

 fuse together, and the mixture goes into the gel state. We use gelatin, 

 corn starch, egg white, and flour to form colloidal systems in the gel 

 state to make many of our everyday foods more attractive and tasty. 



Fig. 3.4. From sol to gel. This diagram shows how the particles of a dispersion 



may change from a discontinuous phase to a continuous phase and thus change the 



nature of the mixture from a liquid to a jelly-like solid. 



Changes in viscosity of protoplasm through slight changes in the 

 sol-gel relationship account for many of the activities within a cell. 

 When certain cells are viewed under the microscope, we can see an 

 active movement of the protoplasm as it flows around in the cell. This 

 is known as protoplasmic streaming and is brought about by such 

 changes in viscosity of protoplasm in different parts of the cell. When 

 an animal dies, the body is limp because of the relative fluidity of the 

 protoplasm in the cells. Within a short time, however, the body be- 

 comes rigid ; we say that rigor mortis has set in. The wastes which 

 accumulate in the body of a dead animal bring about a change in the 

 protoplasm from the sol to the gel state ; we say that the protoplasm is 

 coagulated. This causes the stiffening. 



Organic and Inorganic Compounds 



It was once said that organic compounds are those that can be 

 produced only by living cells, whereas inorganic compounds do not 

 need such a background of living matter. As the science of organic 

 chemistry developed, however, it became possible to synthesize organic 



